The siblings are helping to raise money for life saving heart research by taking on a 26.2mile challenge to help beat heart disease.
And their competitive streak will be unleashed as the brother and sister duo will race to see who can finish first.
New pictures show the pair working up a sweat on a photo shoot to launch the campaign.
Louise, who has been in MIC since its first series in 2011, says she is "delighted" to support the campaign, as the charity is very close to her heart.
"We both know first-hand how devastating heart disease can be as our dad unfortunately had a heart attack many years ago," said Louise. "That's why keeping heart healthy and active is so important to me. I think there will be some sibling rivalry as I'm determined to beat Sam and complete MyMarathon before him."
Meanwhile Sam, who joined MIC in 2013, echoes Louise's comments.
He says: "The money we raise will help save lives and support people like our dad who are affected by heart disease, so we want everyone to get behind us and take part. Together we can beat heart disease. I just hope Louise doesn't beat me!"
MyMarathon enables participants to complete the 26.2mile challenge in their own time and wherever they like. To find out more or to sign up for free by 30 April, visit www.bhf.org.uk/mymarathon.
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